If he does get his project file back to working, exporting the audio as saparate wavs is what needs to be done. Then he can use this program:
https://www.videohelp.com/software/AudioMuxer
I use AudioMuxer all the time and it works great. It’s intended for surround sound music, but can be used for movies too. Just used the tools section and all of the converters and muxers are there. There is even a normalizing tool as well that is helpful